Every time a pilot moves the yoke, stick, or rudder pedals, a network of steel cables translates that input into movement at the control surfaces. These cables rarely run in a perfectly straight line from the cockpit to the tail or wingtip — they must turn corners, pass through bulkheads, and change direction dozens of times along the way. Two critical components make those direction changes possible without damaging the cables: fairleads and pulleys. For the aviation maintenance technician (AMT), understanding the construction, function, and precise inspection criteria for both is not just a matter of passing the FAA knowledge test — it is a fundamental airworthiness responsibility that directly affects flight safety.
This article covers the design and purpose of fairleads and pulleys, how each interacts with the control cable, and the specific inspection criteria that govern whether each component is serviceable. It also highlights the most commonly tested concepts on the FAA AMT Airframe knowledge examination.
Control Cable Routing: The Big Picture
Aircraft control cables are typically fabricated from corrosion-resistant steel wire strands wound into a flexible assembly. These cables must be routed efficiently through the airframe structure, and the routing almost always requires the cable to change direction or be protected where it passes through a structural member. Left unguided, cables would chafe against structure, fray, and eventually fail. Two devices prevent this: fairleads for very small angular changes in cable direction, and pulleys for larger direction changes that require the cable to turn a corner.
Fairleads: Design and Function
A fairlead is a non-rotating guide that supports or redirects a control cable through very small angles — AC 43.13-1B specifies that a fairlead should not deflect a cable more than 3 degrees from its straight-line path. This 3-degree figure is acceptable-practice guidance from AC 43.13-1B Chapter 7 rather than a numeric limit spelled out in a specific regulation, but it is the standard the AMT applies when evaluating fairlead installations. Fairleads are used at bulkhead openings, floor penetrations, and any other location where the cable must pass through structure or be kept from rubbing against the airframe.
Fairleads are typically made from one of two materials: a smooth plastic (often nylon or a similar low-friction polymer) or a soft aluminum block. The inner surface through which the cable passes is finished smoothly so that the cable can slide or move longitudinally without significant friction. Some fairleads are a simple ring or grommet shape; others are molded blocks with a hole or groove sized to match the cable diameter. The key design principle is that the fairlead does not rotate — the cable slides through or over it. This means friction and wear, though minimized by smooth materials and the shallow deflection angle, are always present to some degree.
Because fairleads are non-rotating, the amount of angular deflection they are permitted to impose on the cable is strictly limited. Exceeding the 3-degree guideline places unnecessary side-loading on the cable, accelerating wear on both the cable strands and the fairlead itself. When a routing requires a deflection larger than 3 degrees, a pulley must be used instead.
Pulleys: Design and Function
A pulley is a rotating wheel with a grooved rim designed to accommodate and redirect a control cable. Because the pulley rotates freely on a bearing or bushing, the cable rolls over the sheave rather than sliding against a fixed surface, dramatically reducing friction and wear even when the direction change is substantial. Pulleys are used wherever the cable must change direction by more than 3 degrees — this includes the large turns at the base of the cockpit controls, at bellcrank assemblies, and throughout the tail section.
Pulleys are typically made from aluminum alloy, though older aircraft may use phenolic (hard plastic) or steel pulleys in certain applications. The groove around the circumference of the pulley — called the sheave — is machined or formed to a specific width and depth that matches the diameter of the cable it serves. A correctly sized groove allows the cable to seat partially into it, providing support and preventing the cable from slipping sideways off the pulley under load. The pulley rotates on a pulley bracket, the structural mount that holds the pulley and its bearing or bushing in place. A separate component, the cable guard (or guard assembly) — a small pin, plate, or formed bracket — prevents the cable from jumping out of the groove during slack conditions or vibration.
Inspection of Fairleads
The AMT must inspect every fairlead in the control system during routine maintenance and rigging inspections. The following conditions make a fairlead unairworthy:
- Wear or grooving: A fairlead that has developed a noticeable groove from cable contact must be replaced. Even a shallow groove can trap the cable and restrict longitudinal movement, adding friction to the control feel or, in a worst case, jamming the cable.
- Cracks or splits: Plastic fairleads can crack from age, UV exposure, or mechanical impact. A cracked fairlead may eventually pinch the cable or fragment and lodge in the cable path.
- Misalignment: The fairlead must be positioned so that the cable approaches and exits at essentially the same angle, within that 3-degree guideline. A fairlead that has shifted or is installed off-axis increases side-loading on the cable.
- Excessive cable deflection: If the cable visibly bends at the fairlead by more than 3 degrees, the routing must be corrected or a pulley substituted.
- Looseness or improper attachment: A fairlead that is not securely attached to the structure can migrate, changing the cable geometry and potentially allowing the cable to contact sharp structural edges.
Inspection of Pulleys
Pulley inspection is more involved than fairlead inspection because the pulley is a rotating component with more potential failure modes. Inspectors check for the following:
- Bearing condition: The pulley must rotate freely and smoothly on its bearing or bushing. Roughness, binding, or excessive play in the bearing indicates wear that can cause the pulley to resist rotation, effectively converting it to a non-rotating guide — which adds friction and can damage the cable strands.
- Groove wear: The sheave groove should be inspected for wear. A groove that has been worn flat, undercut, or widened will no longer support the cable correctly. If the cable rides too deep in an undercut groove, it can jam; if the groove is too wide, the cable can shift laterally and fray against the groove edges.
- Cracks and damage: The pulley body itself must be free of cracks, nicks, or corrosion that could weaken it or damage the cable.
- Guard or retainer condition: The cable guard must be present, undamaged, and properly positioned. The clearance between the guard and the pulley rim is typically specified by the manufacturer but is generally very small — enough to allow the pulley to rotate freely but not enough to allow the cable to escape the groove under slack conditions.
- Alignment: The pulley must be aligned so that the cable approaches and departs tangentially, without side-loading the cable against the pulley flanges. Misaligned pulleys cause rapid flange and cable wear.
- Evidence of cable contact on flanges: If the cable has been rubbing against the side flanges of the pulley rather than riding in the groove, shiny wear marks will be visible on the flanges. This is a sign of misalignment or incorrect cable tension and must be corrected.
Cable Inspection at Fairleads and Pulleys
The condition of the cable itself at every fairlead and pulley contact point must also be assessed. The FAA specifies limits for broken wires in a given length of cable — for example, in a 7×19 cable, the presence of broken wires in a short section may require cable replacement, depending on the number and location of breaks. Fairleads and pulleys are among the highest-wear locations in any cable system, so the inspector should pay particular attention to the cable strands at these points. Frayed, kinked, or corroded sections are grounds for cable replacement regardless of the overall cable condition elsewhere.
Why It Matters
The consequences of overlooking worn fairleads or defective pulleys range from degraded control feel to complete loss of a control surface. A jammed or heavily worn pulley can cause a control to become stiff, slow, or asymmetric — problems that may be subtle on the ground but dangerously apparent in flight. A pulley that seizes completely converts the system to a sliding friction system, dramatically increasing cable loads and the force required to move the controls. A cable that escapes a pulley groove due to a missing guard can jam between the pulley and the bracket, locking the control entirely. These are not theoretical risks — they have been documented in NTSB accident records and are exactly why the FAA requires systematic inspection of every component in primary and secondary control cable systems.
Key Numbers and Rules
- Maximum fairlead deflection: 3 degrees from straight-line cable path, per AC 43.13-1B guidance.
- When to use a pulley vs. a fairlead: Any direction change greater than 3 degrees requires a pulley.
- Pulley materials: Typically aluminum alloy; phenolic or steel in older or specialized applications.
- Fairlead materials: Smooth plastic (nylon) or soft aluminum.
- Cable guard clearance: Must be small enough to prevent cable from leaving the groove; exact value per manufacturer's data.
- Broken wire limits: Governed by aircraft maintenance manual and FAA guidance; broken wires at wear points are a critical finding.
Common Test Traps
- Mixing up the 3-degree rule: Students sometimes misremember the maximum fairlead deflection angle or confuse it with other rigging tolerances. The guideline is specifically 3 degrees for fairleads; anything greater requires a pulley.
- Assuming pulleys rotate freely in all conditions: A seized or rough-bearing pulley effectively becomes a fixed guide, and this failure mode is a common exam question about why pulley bearing condition matters.
- Ignoring the cable guard: Test questions may describe a pulley assembly with a missing or bent guard and ask whether the installation is airworthy. A missing guard is an unairworthy condition regardless of pulley condition.
- Confusing flange wear vs. groove wear: Shiny wear marks on the pulley flanges (sides) indicate misalignment; wear in the groove bottom indicates prolonged cable contact from correct but heavy use. Both are inspection findings but point to different root causes.
- Overlooking cable condition at contact points: The AMT must inspect the cable at the fairlead and pulley, not just between them. The highest wear on a cable occurs at these interfaces, and a cable that looks fine elsewhere may be critically worn at a pulley groove.